1. Field of the Invention
The present invention relates to a process and an apparatus system for producing a toner with a given particle size for developing electrostatic images, by efficiently pulverizing and classifying solid particles containing a binder resin.
2. Related Background Art
In image forming processes such as electrophotography, electrostatic photography and electrostatic printing, a toner is used to develop an electrostatic image.
As a process for producing an end product by pulverizing and classifying starting solid particles in the production of a toner for developing electrostatic image in which the end product is required to be of fine particles, the process as shown in a flow chart in FIG. 6 is commonly used. This process comprises melt-kneading given starting materials such as a binder resin, a coloring agent as exemplified by a dye, a pigment and a magnetic material, cooling the kneaded product to solidification, followed by pulverization to obtain pulverized solid particles as a pulverized feed material.
The pulverized feed material is constantly fed to a first classifying means and classified therein. A classified coarse powder maily comprised of coarse particles having a particle size above a prescribed range is fed to a pulverizing means and pulverized therein, and then the pulverized product is again fed back to the first classifying means.
The powder mainly comprised of particles having a particle size within other prescribed range and particles having a particle size below the prescribed range is fed to a second classifying means, and classified into a median powder mainly comprised of particles having the prescribed particle size and a fine powder mainly comprised of particles having a particle size below the prescribed particle size.
For example, in order to obtain particles having, for example, a volume average particle diameter of 8 .mu.m and also a coefficient of variation of number distribution, represented by A as defined later, of 33, the starting material is pulverized to powder with a given average particle diameter and classified, using a pulverizing means such as an impact mill or jet mill quipped with a classifying mechanism for removing coarse powder, and the pulverized feed material from which the coarse powder has been removed is passed to another classifier, where a fine powder is removed to give the desired median powder.
The volume average particle diameter herein referred to is a measurement obtained by a Coulter counter Type TA-II, available from Coulter Counter, Inc. (U.S.A.), using an aperture of 100 .mu.m.
Such conventional processes have the following problems. Particles from which coarse particles with a particle size above a prescribed range have been completely removed must be fed to the second classifying means provided for the purpose of removing the fine powder, and hence the pulverizing means necessarily bears a greater load, bringing about a smaller throughput. In order to completely remove the coarse particles with a particle size above a prescribed range, it tends to result in excessive pulverization after all. This leads to the problem that a phenomenon such as a lowering of the yield is caused in the subsequent second classifying means for removing the fine powder.
In respect of the second classifying means provided for the purpose of removing the fine powder, an aggregate constituted of ultrafine particles may be produced in some instances, and it is difficult to remove the aggregate as a fine powder. In such an instance, the aggregate may be mixed into the end product, resulting in a difficulty to obtain a product having a precise particle size distribution. Moreover, the aggregate may be disintegrated in a toner into ultrafine particles to give a cause to lower image quality.
Even if the desired product having a precise particle size distribution can be obtained using the conventional method, its process becomes complicated to cause a lowering of the yield of classification, necessarily resulting in a poor production efficiency and a product of high cost. This tendency increases with a decrease in the given particle size.
This tendency more increases when the volume average particle diameter is 10 .mu.m or less.
Japanese Patent Application Laid-open No. 63-101859 (corresponding to U.S. Pat. No. 4,844,349) discloses a process and an apparatus for producing a toner, comprising a first classifying means, a pulverizing means and a multi-division classifying means used as a second classifying means. It, however, is sought to provide a process and an apparatus system for efficiently producing a toner having a volume average particle diameter of 10 .mu.m or less.